1. Field of the Invention
The present invention relates to organic light emitting devices (OLEDs). In particular, the present invention relates to a driver circuit for an OLED pixel that has a current mode and a voltage mode.
2. Description of Prior Art
An OLED device typically includes a stack of thin layers formed on a substrate. In the stack, a light-emitting layer of a luminescent organic solid, as well as adjacent semiconductor layers, are sandwiched between a cathode and an anode. The light-emitting layer may be selected from any of a multitude of fluorescent organic solids. Any of the layers, and particularly the light-emitting layer, also referred to herein as the emissive layer or the organic emissive layer, may consist of multiple sublayers.
In a typical OLED, either the cathode or the anode is transparent. The films may be formed by evaporation, spin casting or other appropriate polymer film-forming techniques, or chemical self-assembly. Thicknesses typically range from a few monolayers to about 1 to 2,000 angstroms. Protection of an OLED against oxygen and moisture can be achieved by encapsulation of the device. The encapsulation can be obtained by means of a single thin-film layer situated on the substrate, surrounding the OLED.
High resolution active matrix displays may include millions of pixels and sub-pixels that are individually addressed by the drive electronics. Each sub-pixel can have several semiconductor transistors and other IC components. Each OLED may correspond to a pixel or a sub-pixel, and therefore these terms are used interchangeably hereinafter.
In an OLED, one or more layers of semiconducting organic material may be sandwiched between two electrodes. An electric current is applied to the device, causing negatively charged electrons to move into the organic material(s) from the cathode. Positive charges, typically referred to as holes, move in from the anode. The positive and negative charges meet in the center layers (i.e., the semiconducting organic material), combine, and produce photons. The wave-length—and consequently the color—of the photons depends on the electronic properties of the organic material in which the photons are generated.
The color of light emitted from the organic light emitting device can be controlled by the selection of the organic material. White light may be produced by generating blue, red and green lights simultaneously. Specifically, the precisely color of light emitted by a particular structure can be controlled both by selection of the organic material, as well as by selection of dopants in the organic emissive layers.
Pixel driver circuits can be configured as either current sources or voltage sources to control the amount of light generated by the OLED diode in an active matrix display. AMOLED microdisplays may require low amounts of current to generate light, especially when using analog gray scale rendition techniques. OLEDs may be driven in current mode due to the linear dependence of luminance on operating current. For low light level applications, a typical OLED microdisplay pixel current may be in the range of 10's to 100's of picoamps. A long channel transistor may be used to generate the output current.